json/doc/mkdocs/docs/api/basic_json/value.md

nlohmann::basic_json::value

// (1)
template<class ValueType>
ValueType value(const typename object_t::key_type& key,
                ValueType&& default_value) const;

// (2)
template<class KeyType, class ValueType>
ValueType value(KeyType&& key,
                ValueType&& default_value) const;

// (3)
template<class ValueType>
ValueType value(const json_pointer& ptr,
                const ValueType& default_value) const;

1. Returns either a copy of an object's element at the specified key key or a given default value if no element with key key exists.

   The function is basically equivalent to executing

   try {
      return at(key);
   } catch(out_of_range) {
      return default_value;
   }
   

2. See 1. This overload is only available if KeyType is comparable with #!cpp typename object_t::key_type and #!cpp typename object_comparator_t::is_transparent denotes a type.

3. Returns either a copy of an object's element at the specified JSON pointer ptr or a given default value if no value at ptr exists.

   The function is basically equivalent to executing

   try {
      return at(ptr);
   } catch(out_of_range) {
      return default_value;
   }
   

!!! note "Differences to at and operator[]"

- Unlike [`at`](at.md), this function does not throw if the given `key`/`ptr` was not found.
- Unlike [`operator[]`](operator[].md), this function does not implicitly add an element to the position defined by
 `key`/`ptr` key. This function is furthermore also applicable to const objects.

Template parameters

KeyType

A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17). ValueType

type compatible to JSON values, for instance #!cpp int for JSON integer numbers, #!cpp bool for JSON booleans, or #!cpp std::vector types for JSON arrays. Note the type of the expected value at key/ptr and the default value default_value must be compatible.

Parameters

key (in)

key of the element to access

default_value (in)

the value to return if key/ptr found no value

ptr (in)

a JSON pointer to the element to access

Return value

1. copy of the element at key key or default_value if key is not found
2. copy of the element at key key or default_value if key is not found
3. copy of the element at JSON Pointer ptr or default_value if no value for ptr is found

Exception safety

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

Exceptions

1. The function can throw the following exceptions:
   • Throws type_error.302 if default_value does not match the type of the value at key
   • Throws type_error.306 if the JSON value is not an object; in that case, using value() with a key makes no sense.
2. See 1.
3. The function can throw the following exceptions:
   • Throws type_error.302 if default_value does not match the type of the value at ptr
   • Throws type_error.306 if the JSON value is not an object; in that case, using value() with a key makes no sense.

Complexity

1. Logarithmic in the size of the container.
2. Logarithmic in the size of the container.
3. Logarithmic in the size of the container.

Examples

??? example "Example (1): access specified object element with default value"

The example below shows how object elements can be queried with a default value.

```cpp
--8<-- "examples/basic_json__value.cpp"
```

Output:

```json
--8<-- "examples/basic_json__value.output"
```

??? example "Example (3): access specified object element via JSON Pointer with default value"

The example below shows how object elements can be queried with a default value.

```cpp
--8<-- "examples/basic_json__value_ptr.cpp"
```

Output:

```json
--8<-- "examples/basic_json__value_ptr.output"
```

See also

• see at for access by reference with range checking
• see operator[] for unchecked access by reference

Version history

1. Added in version 1.0.0. Changed parameter default_value type from const ValueType& to ValueType&& in version 3.11.0.
2. Added in version 3.11.0.
3. Added in version 2.0.2.